1. Field of the Invention:
This invention relates to a semiconductor laser having a structure, which is effective to control the transverse mode of laser oscillation and reduce the level of the threshold current, by the use of a crystal growth technique for the formation of thin films such as molecular beam epitaxy or metal-organic chemical vapor deposition.
2. Description of the Prior Art:
Recently, single crystal growth techniques for the formation of thin films, such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc. have been developed which enable the formation of thin film growth layers having a thickness of as thin as approximately 10.ANG.. The development of such techniques although these significantly thin films have not yet been produced by liquid phase epitaxy (LPE), allowed thin films to be applied to lasers, resulting in laser devices exhibiting new laser effects and/or superior laser characteristics. A typical example of these new laser devices is a quantum well (QW) laser, which is produced based on the fact that quantization levels are established in its active layer by reducing the thickness of the active layer from several hundred .ANG. to approximately 100.ANG. or less, and which is advantageous over conventional double heterostructure lasers in that the threshold current level is low and the temperature and transient characteristics are superior. Such a quantum well laser is described in detail in the following papers:
(1) W. T. Tsang, Applied Physics Letters, vol. 39, No. 10 pp.786 (1981), PA1 (2) N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11, pp. 7211 (1982), and PA1 (3) H. Iwamura, T. Saku, T. Ishibashi, K. Otuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, pp. 780 (1983).
As mentioned above, the single crystal growth techniques, such as molecular beam epitaxy or metal-organic chemical vapor deposition, have resulted in the practical use of high quality semiconductor lasers having a new multiple-layered structure. However, these semiconductor lasers are deficient in that the stabilized transverse mode of laser oscillation cannot be attained due to the multiple-layered structure.
One of the most important points requiring improvement in other conventional semiconductor lasers which are in practical use, is stabilization of the transverse mode of the laser oscillation. A contact stripe geometric laser, which was developed in the early stage of laser development, has a striped electrode to prevent electric current injected from transversely expanding, and attains laser oscillation in a zero order mode (i.e., the fundamental transverse mode) upon exceeding the threshold current level, due to the fact that gains required for laser oscilation are greater than losses within the active region underneath the stripe region, while the said contact stripe geometric laser produces laser oscillation in an expanded transverse mode or a higher-order transverse mode with an increase in the injection of current beyond the threshold current level, because carriers which are injected into the active layer spread to the outside of the striped region resulting in expanding the high gain region. Due to such an unstable transverse mode and dependency of the transverse mode upon the amount of injected electric current, the linear relationship between the injected electric current and the laser output decreases. Moreover, the laser output resulting from pulse modulation is unstable so that the signal-noise ratio is reduced and its directivity becomes too unstable to be used in an optical system such as optical fibers, etc. In order to overcome the above-mentioned practical drawbacks of contact stripe geometric lasers, a variety of structures for semiconductor lasers of GaAlAs and/or InGaAsP systems have been already produced by liquid phase epitaxy, which prevent not only electric current but also light from transversely expanding thereby attaining stabilization in the transverse mode. However, most of these semiconductor lasers can only be produced by the growth of thin film layers on a grooved substrate, a mesa substrate or a terraced substrate based on a peculiarity of the liquid phase epitaxy, typical examples of which are channel-substrate planar structure injection lasers (CSP lasers) (K. Aiki, M. Nakamura, T. Kuroda and J. Umeda, Applied Physics Letters, vol. 30, No. 12, pp. 649 (1977)), constricted double heterojunction lasers (CDH lasers) (D. Botez, Applied Physics Letters, vol. 33, pp. 872 (1978)) and terraced substrate lasers (TS lasers) (T. Sugino, M. Wada, H. Shimizu, K. Itoh, and I. Teramoto, Applied Physics Letters, vol. 34, No. 4, (1979)). All of these lasers can be only produced utilizing anisotrophy of the crystal growth rate, but can not be produced by the use of a crystal growth technique such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MO-CVD).
FIG. 1 shows a conventional semiconductor laser of a GaAlAs system which achieves a stabilized transverse mode, which can be produced as follows: On an n-GaAs substrate 1, an n-GaAs buffer layer 1', an n-Ga.sub.0.7 Al.sub.0.3 As cladding layer 2, an n-GaAs active layer 3, a p-Ga.sub.0.7 Al.sub.0.3 As cladding layer 4 and a p-GaAs cap layer 5 are successively grown by molecular beam epitaxy, followed by a successive vacuum evaporation of each of Al, Zn and Au to form an electrode layer thereon which is then subjected to a photolithography treatment to form a striped electrode layer 20. The resulting semiconductor layer is etched by an Ar ion etching technique using the striped electrode layer 20 as a masking means to thereby form a striped portion 10 which serves as an optical waveguide. The cladding layer 4 on both sides of the striped portion 10 has a thickness of approximately 0.3 .mu.m. The striped electrode layer 20 is heated to be an alloy. Then, a SiO.sub.2 film 6 is formed on the cladding layer 4 on both sides of the striped portion 10. A Cr/Au electrode 8 and an AuGe/Ni electrode 7 are formed on the p-side (i.e., the striped portion 10 and the SiO.sub.2 film 6) and the n-side (i.e., the bottom of the substrate 1), respectively, resulting in a semiconductor laser. Such a semiconductor laser exhibits stabilized characteristics. However, since the difference in the built-in refractive index in the optical waveguide is dependent upon the accuracy of the depth of the etching by an Ar ion beam etching technique, the control of the difference in the refractive index is so difficult that reproducible oscillation in the fundamental transverse mode cannot be attained. Moreover, control of the oscillation at a high output power is difficult because the difference in the refractive index must be minimized to attain oscillation at a high output power.